The invention relates to removal of toxic substances from water, and is particularly directed to removal of toxic metals from agricultural tile drain water. Typically, such substances are natural minerals which are picked up by the water in leaching through the ground when the water is used for irrigation, and these substances include selenium and molybdenum.
The process of the invention is particularly concerned with removal of selenium from the drain waters produced from irrigation of soil containing natural alluvial deposits of selenium. In some cases there will also be, or there will alternatively be, molybdenum present.
In this specification and the accompanying claims, the term "selenium" or "molybdenum" is intended to refer to all soluble compounds of each of these metals, as well as the elemental metals themselves.
A particularly acute problem of selenium buildup (and also molybdenum) has occurred in the drain waters from irrigation of the San Joaquin Valley in Calif. Subterranean drains are necessary there to prevent the buildup of salts in the perched water layer which, if not drained, would build up to a level whereby the saline layer would reach the plant roots, with the resultant loss of that land for useful agriculture.
The perched water layer in many areas of the San Joaquin Valley percolates through selenium-bearing soils, with the result that the selenium content reaches toxic levels. This is particularly true when the tile drain water is retained in an evaporation pond or reservoir, with consequent concentration of the brine. The seleniferous drain water cannot be disposed of safely or legally without prior removal of the selenium.
San Joaquin Valley tile drain water is typified by the data given in Table I. Selenium concentration and molybdenum content and the total dissolved solids (TDS) are all too high to be recycled for irrigation or to be dumped into a river or ocean bay. Further, if stored in an open reservoir, the tile water will poison birds and fish and other marine life, eventually finding its way into the human food chain.
TABLE I ______________________________________ FIREBAUGH TILE WATER ANALYSIS (Firebaugh Irrigation District, San Joaquin Valley, Calif.) ______________________________________ pH 8.0 Carbonate (CO.sub.3), mg/L *1 Bicarbonate (HCO.sub.3), mg/L 150 Chloride (Cl), mg/L 650 Sulfate (SO.sub.4), mg/L 4000 Calcium (Ca), mg/L 380 Magnesium (Mg), mg/L 300 Sodium (Na), mg/L 1600 Iron (Fe), mg/L *0.1 Manganese (Mn), mg/L *0.01 Nitrate (NO.sub.3), mg/L 74 Fluoride (F), mg/L 0.3 Potassium (K), mg/L 2 Selenium (Se), mg/L 0.28 Specific Conductance (EC), 8600 micromhos/cm at 25.degree. C. Hardness as CaCO.sub.3, mg/L 2200 Hardness as CaCO.sub.3, gpg 130 Total Dissolved Solids (TDS), mg/L 7200 ______________________________________ *Less than
Prior selenium removal systems and methods have been concerned primarily with mine waters and similar drain waters. Many efforts have been made, unsuccessfully, to remove selenium from agricultural drain waters. It is known that selenium plus 6 valence can be reduced to selenium plus 4, which occurs as selenite. This can be reduced to elemental selenium at a valence of 0. This can be reduced further to a valence of minus 2 or selenide. A complicating factor in the agricultural drain waters, aside from the selenium content, is the presence of nitrates originating from nitrogenous fertilizers. These nitrates apparently tend to consume reducing agents by themselves being reduced to nitrites.
U.S. Pat. No. 4,405,464, issued to Kerr-McGee Nuclear Corporation, is pertinent to this invention in that it discloses a process for removing selenium from mine waters. However, the disclosed removal process is not applicable to the removal of selenium or molybdenum from agricultural irrigation drain waters, since it does not deal with the contained nitrate in the agricultural drain waters. Further, the Kerr-McGee process involves passage of the liquid solution up through a column of iron particles. This would involve a very large amount of iron, with a limited amount of water passing through it, and the process thus would appear not to be economically feasible for purposes of the present invention.
In the Kerr-McGee patent, zinc granules gave a modest reaction in attempting selenium recovery. Copper, manganese, magnesium and aluminum powders gave very modest selenium reaction.
Mayenkar U.S. Pat. No. 4,565,633 discloses a process for removal of dissolved heavy metals from waste effluents. The process disclosed in the patent is somewhat similar to that of the Kerr-McGee patent. Mayenkar suggested the use of coarse iron filings (optimally 35 to 45 mesh, U.S. Standard Sieve), in a bed into which the aqueous solution was introduced. A long contact time with the iron filings was relied upon in the disclosed process. A pilot plant was actually built in accordance with the teachings of the patent, to treat Firebaugh irrigation effluent, but the project was unsuccessful.
In U.S. Pat. No. 4,026,797, nickel, cobalt and iron gave reasonable recoveries of selenium at 180 psi autoclaving, at pH less than 3. Sodium sulfide and 5 grams per liter iron as ferric sulfate, also with autoclaving, was reported as giving a reasonable selenium conversion.
In U.S. Pat. No. 4,497,654, chromous sulfate reduction was used in metal sulfate solutions to effectively reduce 10 milligrams per liter selenium at 60.degree. C. This would be unworkable, as the resultant chromium input would be almost as deleterious as the original selenium.
In U.S. Pat. No. 4,544,541, sodium borohydride at 5 grams per liter was effective in reducing 18 milligrams per liter of selenium down to about 1 milligram per liter. This could be optimized to be effective, but is much too expensive to be practical.
An important object of the present invention is to efficiently and economically remove selenium, and other toxic heavy metals such as molybdenum, from agricultural tile drain waters such as those found in the San Joaquin Valley of Calif.